Gravure printing is a method that uses the Intaglio process in which the image to be printed consists of depressions etched or engraved usually to different depths, on an engravable copper plated cylinder. Slightly viscous solvent inks are applied to the entire surface and a metal doctor blade removes the excess ink from the non-printing surface. In a typical process, engraving is performed on the copper plated cylinder, which is subsequently chrome plated to minimize wear.
In order to obtain a consistent quality of engraving, the hardness and crystal structure of the plated copper deposit is of paramount importance. Successful engraving is typically only obtained at a copper hardness of more than 200 Vickers Hardness (HV). At lower values, the engraved cell pattern loses definition. In addition, if the hardness of the deposit exceeds 240 HV, the lifetime of the diamond styli used to engrave the cylinders during electronic engraving may be reduced. Because of these factors, it is important to control the hardness of the plated copper deposits to within the desired range (200–240 HV).
By the use of suitable organic additives, it is possible to produce “as plated” copper deposits within this hardness range, but over time, the deposit “self-anneals” at room temperature and the hardness of the copper deposit falls to a value typically of between 140 and 170 HV. Annealing is the tendency of the hardness of the copper deposit to decrease with time as a result of changes in crystalline size, texture, microdeformations and dislocations within the copper deposit. Also, the depth of immersion of the cylinder during plating (i.e., partial immersion or full immersion) can affect the propensity of the deposit towards self-annealing.
Previously, a combination of organic additives has been used to produce a stable hardness in partially immersed cylinders, as described in U.S. Pat. No. 4,334,966 to Beach et al. and for fully immersed cylinders, the combination of additives described in U.S. Pat. No. 4,781,801 to Frisby has been used. Subsequent to these inventions, U.S. Pat. No. 5,417,841 to Frisby described a combination of an alkoxythio compound (consisting of alkoxylated thiodiglycol), and a sulphonated sulphurised hydrocarbyl compound to stabilised the hardness of copper deposited from both partially and completely immersed cylinders in copper sulphate based plating electrolytes. However, additional advancements are still needed to provide a copper plating deposit that has a suitable copper deposit.
Electronic engraving is a means of transferring an image for printing to a copper electroplated cylinder by directing a diamond-pointed stylus to form as many as 4,000 ink-receiving impressions every second. This technique requires copper deposits of very definite properties to prevent engraving defects and costly damage to the expensive equipment. It is essential that the deposited copper have a homogeneous fine-grained crystal structure that is free of nodulations and occlusions with excellent ductility and uniform hardness. A critical factor is the control and uniformity of hardness since the stylus pressures are set with references to a given Vickers hardness value and if this is not uniform over the entire surface, it will result in smearing or ripping of the deposit and badly defined impressions for printing. Examples of electronic engraving apparatus are described in U.S. Pat. No. 4,450,486 to Buechler and U.S. Pat. No. 6,348,979 to Flannery et al., the subject matter of each of which is herein incorporated by reference in its entirety.
Another potential problem in fabricating gravure and other printing cylinders is the difficulty in producing cylinders having surface properties that are identical from cylinder to cylinder. Surface defects such as roughness, pits or spots that are too hard or too soft, result in engraving errors and the subsequent need for repolishing and replating which can be both expensive and time consuming.
As discussed above, gravure printing cylinders may also be plated either partially or fully submerged, wherein the deposition rate is related to the immersion depth. An important advantage realized by increasing the immersion depth is a decrease in plating time, which has obvious economic advantages.
When a cylinder is plated partially immersed, i.e. to about 30% of its diameter, as compared to a cylinder that is plated totally submerged, the deposit characteristics are influenced by the fluctuations of the current and composition differences in the cathode film. In any event, plating baths are known to perform differently with respect to the immersion depth. The principal problem in this regard is annealing. This problem of recrystallization (annealing) can be characteristic of totally submerged cylinder operations when using a bath designed for partial immersion.
There remains a need in the art for an acid copper plating process which can be used to deposit a copper layer of uniform hardness and stability, which is suitable for electronic engraving, on rolls which are plated while partially submerged or completely or nearly completely submerged in the plating bath.
In addition to the requirement for stable hardness of copper deposit, it is an important requirement to plate the copper deposits as rapidly as possible. Copper sulphate based electrolytes have limitations as to the maximum rate of deposition due to limitations of solubility. Copper methanesulphonate is much more soluble than copper sulphate allowing higher copper concentrations in the electrolyte which in turn allows higher plating rates. A further limitation of sulphate based copper electrolytes is the maximum anode current density which may be applied. Above a certain threshold anodic current density, anode polarisation prevents effective operation of the process.
Methanesulphonate based electrolytes are much less prone to anode polarisation as is shown in FIG. 1. It can be seen from this FIGURE that in static conditions, phosphorised copper anodes will not sustain a continuous current of more than 2.4 A/dm2 whereas in a methanesulphonate electrolyte, a continuous current of 8.7 A/dm2 can be sustained. However, whereas plating from a methanesulphonate electrolyte has many advantages in terms of plating speed and resistance to anode polarisation, the problem of deposit self-annealing remains. In addition, the additive combination disclosed in U.S. Pat. No. 5,417,841 to Frisby does not prevent deposit self-annealing in methanesulphonate baths so additional improvements are still needed
The present invention is directed to a method for producing copper deposits of stable hardness that are free from self-annealing, at high speed. The invention is particularly directed to the high speed application of copper to gravure cylinders. The invention is also usable in the high speed plating of copper in other applications where a deposit of stable hardness is required.